Prior to the present invention as shown by the copending application of Griffing et al., Ser. No. 675,915, filed Nov. 28, 1984, now U.S. Pat. No. 4,677,049 for SPIN CASTABLE PHOTOBLEACHABLE LAYER FORMING COMPOSITIONS, assigned to the same assignee as the present invention and incorporated herein by reference, diarylnitrones were utilized in combination with inert organic polymer binders to make contrast enhancement layer- forming compositions. Although the diarylnitrone photobleachable layer-forming compositions of Griffing, et al. have been found to be valuable materials for use in near UV (350-450 nm) contrast enhanced lithography, recent interest has developed in developing photoresists for use in the mid-UV imaging areas such as 300-350 nm. It has been found that, although arylnitrones can be totally bleached under mid-UV light, the diaryloxaziridines are somewhat absorbant towards mid-UV light and undergo secondary photochemical reactions. As a result, diarylnitrone-based contrast enhancing formulations are generally used in near-UV photolithography. For example, simple diarylnitrones such as .alpha., N-diphenylnitrone (.lambda.313 nm, .epsilon.==22,000 L/mole cm), or .alpha.-anisyl-N-phenylnitrone (.lambda.329 nm, .epsilon.=23,000 L/mole cm) absorb in the desired regions and also appear to bleach efficiently in dilute solutions. However, when cast in films of a copolymer such as styrene and allyl alcohol, the films exhibit poor bleaching characteristics. Thus, the films initially bleach rapidly, but after reaching a transmission maximum of 60% or less, transmission can fall off rapidly if photolysis is continued. One possible explanation is that the absorption spectrum of the oxaziridine photoproduct tails sufficiently into the 300 nm region to undergo secondary photochemical reactions. It is desirable, therefore, to find nitrone systems having suitable spectral characteristics which do not undergo secondary reactions under the applied illumination conditions.
The present invention is based on our discovery that .alpha.-aryl-N-alkylnitrones and dialkylnitrones, as defined hereinafter, possess spectral and photochemical properties which make them particularly well suited for use as contrast enhancement materials for mid-UV (300-365 nm) applications. It has been further found that when these alkylnitrones are formulated with certain inert organic polymers in the presence of a solvent such as water, a water soluble organic solvent, a mixture of water and such water soluble organic solvent, or a water immiscible organic solvent, spin castable mixtures can be made which are suitable for forming in situ, a photobleachable contrast enhancement layer (CEL) on the surface of a photoresist. It has been further found that a suitable organic solvent can be used to strip the photobleached (CEL) from the photoresist surface when a water insoluble inert organic polymer is used in the spin castable mixture. Alternatively, total removal of the CEL also can be achieved simultaneously during development of the photoresist under aqueous conditions when a water soluble inert organic binder is used.